Nanocrystals of semiconductor material typically size in range from about 1 to about 100 nanometers in diameter and have unique optical properties not found in the bulk materials. One particularly important property of such nanocrystals is the dependence of the emission wavelength on the size of the particle.
A number of methods for the formation of core nanocrystals from metal-anion binary salts are known in the art. These methods can generally be divided into classes based on the type of reactants employed and the presumed mechanism that arises based on how the oxidation states of the reactants compare. In the first approach, the metal and nonmetal components that are reacting with each other are both provided in their neutral atomic form. For example, Murray, et al., J. Am. Chem. Soc., 1993, 115: 8706, described the reaction of dimethylcadmium (Me2Cd) and trioctylphosphine selenide (TOPSe), which release neutral cadmium (Cd0) and selenium (Se0) atoms in solution respectively, so that no electron transfer is required to make their oxidation states match. Because the reactants are in suitable form to react with each other, this situation is considered a ‘match’ of oxidation states: neither needs to be oxidized or reduced for a reaction to occur, and no net electron imbalance results. Such reactions generally proceed very rapidly, because the cadmium and selenium atoms react instantly upon collision to form cadmium selenide (CdSe). In a second category, the metal and nonmetal components are both provided in their ionic forms. For example, Peng, et al., J Am. Chem. Soc., 2001, 123:183, described the preparation of CdSe and cadmium telluride (CdTe) using cadmium oxide (CdO) as the cadmium ion source, in the presence of TOPO and a phosphonic acid ligand, such as hexylphoshonic acid (HPA), octylphosphonic acid (OPA) or tetradecylphosphonic acid (TDPA). Cadmium salts release Cd2+ ions in solution, while non-metal precursors such as bis(trimethylsilyl)sulfide (TMS2S) release S2−, in solution. These reactions also proceed very rapidly, since the cadmium and sulfur ions can also react instantly to form cadmium sulfide (CdS). This reaction type is also considered a ‘match’, because again no oxidation or reduction of either species is required, and they can react in an appropriate stoichiometry to produce a neutral product.
In each of these categories, the extreme reactivity of the intermediates toward each other makes it difficult to control the particle size, particle yield and particle size dispersity. The reacting species, once released in solution, will react at a diffusion-controlled rate, or very nearly that fast. In some instances, the use of ligands or solvents may slow the reaction somewhat, but these approaches have not provided a general approach to controlling particle formation. Because the reactions tend to be so fast, they can be difficult to control. In particular, for example, it is often impossible to prevent such reactions from starting new nanocrystals (referred to as nucleation), which can make it difficult to control a reaction that is intended to add a shell to an existing nanocrystal. It is typically necessary to form a semiconductor shell on a nanocrystal for use in applications of interest, since the shell greatly enhances the chemical and photo-stability of the nanocrystal core. The shell is usually made of a different and complementary semiconductor material from the underlying core nanocrystal; thus if the shell-forming reaction results in nucleation, it forms new nanocrystals with a different composition from what is desired mixed in with the desired ones, and it is extremely difficult to separate the nanocrystals once they are formed as a mixture.
In a third approach, mismatched precursors may be chosen such that one precursor provides a neutral atom in solution under the reaction conditions, while the other precursor provides an ionic atom. For example, a mixture of cadmium alkylphosphonate, which is a source of Cd2+ ions, and trioctylphosphine selenide (TOPSe), which is a source of Se0, might be employed to provide mismatched precursor atoms. Such precursors cannot react to form a neutral species unless an electron transfer agent is present to adjust the oxidation state of one of the reactive species to provide ‘matched’ species capable of undergoing reaction. For example, a reductant could be used to add electrons to Cd2+ to provide two non-ionic species (i.e., Cd0 and Se0), or it could add electrons to Se0 to provide two ionic species (i.e., Cd2+ and Se2−). Either way, once the atomic species are ‘matched’, their reaction can proceed, but the reaction cannot proceed without such an electron transfer agent. Alternatively, two ionic species having the same charge (i.e., two cations or two anions) would also be ‘mismatched.’ For example, mismatched precursors that provide two cationic species could be used, where one species is reduced to provide an anionic species capable of undergoing a ‘matched’ reaction. For example, Se2+ or Se4+ could be reduced to provide selenide anion Se2−, which could undergo reaction with a metal cation species, such as Cd2+. In another example, two cationic species could both be reduced to neutral species.
In another example, an oxidant could be used as the electron transfer agent, in a reaction between a neutral species and an anionic species. For example, Cd0 and Se−2 could be used as mismatched precursors, wherein an oxidant is used to oxidize Se−2 to Se0, giving two neutral species capable of undergoing a ‘matched’ reaction. The need for this electron transfer process and agent has been largely overlooked: because of the small scale and the complexity of the reactions involved, the role of the electron transfer agent is often performed by serendipitous impurities either present in starting materials or accidentally generated in situ. Some reactions having added electron transfer agents have been reported: for example, Zehnder and Treadway (U.S. Pat. No. 7,147,712) described the use of a promoter, which could be oxygen or a reducing agent, to promote and control nucleation and accelerate crystal growth. A single reductant was added to initiate nucleation (initial formation) and facilitate growth of the quantum dots once nucleation had occurred. This approach provided control over the particle yield and over the ultimate particle size. However, because the same reductant was used for both the nucleation and growth phases, separation of the two phases could be achieved only by indirect means.
There remains a need in the art for improved methods for manufacturing nanocrystal products in a high product yield and with a high level of control over particle size and particle dispersity, and also a need for separately controlling the nucleation and growth phases of nanocrystals.